1. Field of the Invention
The present invention relates to a squelch circuit and a communication apparatus used with the same having low power consumption in a squelch detecting circuit and preventing a lack of a squelch signal by using a transmission control signal, in the case where analog operating characteristics of the squelch detecting circuit are insufficient at detecting the existence of a signal on the communication line or where power consumption causes disadvantages in the squelch detecting circuit.
2. Description of the Related Art
In a circuit on a communication apparatus, it is known that the apparatus is applied with a squelch detecting circuit for generating a squelch signal, indicating the existence of a signal on a communication line, and the generated squelch signal is applied to a communication control circuit to control a transmission or a reception operation.
FIG. 1 is a view showing a block diagram of a communication apparatus 200 having a squelch detecting circuit 203 in related art. A main portion according to the squelch circuit in the communication apparatus 200 of FIG. 1 is provided with an input-output control unit 201, a communication control unit 202, and a squelch detecting circuit 203.
A transmission data line and the like are provided from the communication control unit 202 to the input-output control unit 201, and a transmission control signal and data are transmitted. On the other hand, a received data line is provided from the input-output control unit 210 to the communication control unit 202, and received data and a reception control signal are received.
A communication line (or wireless) is provided from the input-output control unit 201 to an external apparatus, and is connected to the squelch detecting circuit 203. An output terminal of the received data line is also connected to the squelch detecting circuit 203, and an output terminal of the squelch detecting circuit 203 is connected to the communication control unit 202.
The squelch detecting circuit 203 generates the squelch signal by applying a signal on the communication line or a data on the received data line between the communication control unit 202 and the input-output control unit 201.
The communication control unit 202 controls the whole communication, and the input-output control unit 201 controls the input and output operations of the communication apparatus.
FIGS. 2A to 2C are views showing a timing chart in the block diagram of the communication apparatus 200 in related art, shown in FIG. 1. In the communication line, a period from t201 to t203 indicates a receiving period, and a period from 205 to t207 indicates a transmitting period. Also, a period from t208 to t210 indicates the receiving period. An ideal squelch signal is shown in FIG. 2B. A communication line (or received data line) shown in FIG. 2A is in a high level in a period from an end time of the receiving period t203 to a start time of the transmitting period t205. And in the same way, it is also in a high level in a period from the end time of the transmitting period t207 to the start time of the receiving period 208, so the squelch signal is generated. In other receiving periods, periods from t202 to t203 and from t208 to t210, and another transmitting period, a period from t206 to t207, the communication line is in a low level, so the squelch signal is not generated. Here, the drawing shows an example in which a squelch control period from t207 to t208 is shorter than that from t203 to t205 in a period except the transmitting period or the receiving period.
In comparison with an ideal squelch signal with ideal transmitting and receiving periods on a communication line or the received data line, a real squelch signal shown in FIG. 2C delays an assert and deassert timings due to analog delay characteristics of the squelch detecting circuit 203 in the period from t203 to t204 (b). Further, in the squelch detecting circuit 203 in which the analog operating characteristics are insufficient, it suffers from the disadvantage that the lack of the squelch signal is generated in the period from t208 to t209 (a) in the case where the duration between the transmission period and the reception period (from t203 to t204) is short. In related art, the current is increased to improve the analog operating characteristics of the squelch detecting circuit 203, and consequently, the delay value of the squelch signal is reduced. However, the amount of current is raised by increasing the frequency to be applied, and as a result, power consumption is further increased.
The above is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2-95030.